[Graphics/3D]: Reset pLastSyncInfo to fix Post and Post2 switch hang issue

 [Graphics/3D]: Reset pLastSyncInfo to fix Post and Post2 switch hang issue

 BZ: 30855

  When HWC enabled, HWC bypass layers will displayed by Post2 interface. But normal
Framebuffer will still use Post interface. In kernel driver, Post2 will call
PVRSRVSwapToDCBuffer2KM, it will append last frame's sync info to current flip command.
But if app switch for Post, the previous buffer maybe is freed and the sync info is wrong.
The patch will clear the pLastSyncInfo and then PVRSRVSwapToDCBuffer2KM will not append
last frame's sync info to the first frame after switch.

Change-Id: If700cd17aa6d55d4e9bf2574e18e078c307ee764
Signed-off-by: Marc Mao <marc.mao@intel.com>
Reviewed-on: http://android.intel.com:8080/42890
Reviewed-by: Xu, Randy <randy.xu@intel.com>
Tested-by: Xu, Randy <randy.xu@intel.com>
Reviewed-by: Wang, Gang A <gang.a.wang@intel.com>
Reviewed-by: buildbot <buildbot@intel.com>
Tested-by: buildbot <buildbot@intel.com>